General communication systems will be first described and, among such communication systems, problems which occur in a communication system using discrete Fourier transform (DFT) spreading will be then described.
First, an orthogonal frequency division multiplexing (OFDM) scheme may be combined with an access scheme such as a time division multiple access (TDMA) scheme, a code division multiple access (CDMA) scheme or a frequency division multiple access (FDMA) scheme, for the access of multiple users. Respective combinations of the OFDM scheme and access schemes have respective merits and demerits. Among them, when the FDMA scheme is applied, it is advantageous that flexibility in allocation of frequency resources and bandwidth efficiency of a system are excellent and an adaptive modulation/demodulation scheme according to channels is applicable. Thus, an OFDM-FDMA (hereinafter, referred to as “OFDMA”) scheme is preferred.
Meanwhile, the OFDM scheme generally has merits that bandwidth efficiency is high and a single-tap channel compensation is possible and thus applies to high-speed data transmission. However, this scheme has problems which should be solved, such as a synchronization problem or a peak-to-average-power ratio (PAPR) problem.
Among various problems, the PAPR problem occurs because a maximum value of a range in which an amplifier of a terminal can linearly amplify a signal is restricted. In the OFDM scheme, since a plurality of carriers overlap one another, a relatively high PAPR occurs. If a maximum power value exceeds a linear range, a transmission signal is distorted and capability deteriorates.
Hereinafter, among various methods for suppressing the PAPR problem, a method for performing DFT spreading will be described.
FIG. 1 is a block diagram of a communication system for spreading and transmitting a transmission signal.
As shown in FIG. 1, the communication system using the spreading of the transmission signal encodes the transmission signal (111), modulates the transmission signal (112), and spreads the transmission signal in a predetermined frequency band (113). In a single carrier FDMA (SC-FDMA) scheme, the DFT spreading is used in the spreading. Thereafter, the spread signal is subjected to FDMA mapping (114), is subjected to OFDM modulation (115), and is converted into an analog signal by a digital-analog converter (DAC) 116. Thereafter, the signal is shifted to a predetermined frequency band by a local oscillator (LO) 117 and is transmitted through a power amplifier (PA) 118 and an antenna 119.
The transmitted signal is received through a channel 120, and is subjected to an inverse process of the process of the transmission side by a reception side. That is, the signal received by an antenna 131 is shifted to a baseband signal by an LO 132 and is converted into a digital signal by an analog-digital converter (ADC) 133. Thereafter, the signal is subjected to OFDM demodulation (134), is subjected to FDMA demapping (135), is equalized/detected (136), is demodulated (137), and is decoded (138).
The SC-FDMA scheme in which the DFT spreading is used in the spreading (113) of the spreading-based system will now be described.
The DFT spreading has a basic concept that, if signals are spread using the DFT having an opposite concept of the inverse fast Fourier transformation (IFFT), the PAPR problems related to overlapping of signals which are generated at the time of performing the IFFT in the OFDM modulation can be solved. Due to such a property, the DFT spreading scheme is also called the SC-FDM scheme. A combination of the SC-FDM scheme and the FDMA scheme is called SC-FDMA.
The DFT spreading concept will now be described in detail.
FIG. 2 is a view illustrating a concept of performing the DFT spreading.
In more detail, FIG. 2 shows a process of spreading input data rows x[0] to x[7] to respective subcarrier areas k and acquiring spread data rows X[0] to X[7] when the size of a DFT spreading band is 8 (N=8). A matrix for performing the DFT spreading and the IDFT which is an inverse process thereof is expressed as follows.
                              W          =                                    1                              N                                      ⁡                          [                                                                                          ω                                              0                        ·                        0                                                                                                                        ω                                              0                        ·                        1                                                                                                  ⋯                                                                              ω                                              0                        ·                                                  (                                                      n                            -                            1                                                    )                                                                                                                                                                                ω                                              1                        ·                        0                                                                                                                        ω                                              1                        ·                        1                                                                                                  ⋯                                                                              ω                                              1                        ·                                                  (                                                      n                            -                            1                                                    )                                                                                                                                                          ⋮                                                        ⋮                                                        ⋱                                                        ⋮                                                                                                              ω                                                                        (                                                      n                            -                            1                                                    )                                                ·                        0                                                                                                                        ω                                                                        (                                                      N                            -                            1                                                    )                                                ·                        1                                                                                                  ⋯                                                                              ω                                                                        (                                                      n                            -                            1                                                    )                                                ·                                                  (                                                      n                            -                            1                                                    )                                                                                                                                ]                                      ,                                  ⁢                  where          ⁢                                          ⁢                      {                                                                                                      DFT                      :                                                                                          ⁢                      ω                                        =                                          ⅇ                                                                        -                          j                                                ⁢                                                                              2                            ⁢                            π                                                    N                                                                                                                                                                                                            IDFT                      :                                                                                          ⁢                      ω                                        =                                          ⅇ                                              j                        ⁢                                                                              2                            ⁢                            π                                                    N                                                                                                                                                                            Equation        ⁢                                  ⁢        1            
where, N denotes the size of data for performing the DFT and IDFT, W denotes a DFT matrix if
  ω  =      ⅇ                  -        j            ⁢                        2          ⁢          π                N            and W denotes an IDFT matrix if
  ω  =            ⅇ              j        ⁢                              2            ⁢            π                    N                      .  
In the spreading-based system, as described above, the PAPR problem can be efficiently solved, but complexity of the signal detection is increased because a reception signal should be detected in consideration of the spreading which is performed at the transmission side.
Particularly, if a virtual multiple input-multiple output (MIMO) technique is used in a communication system using multiple antennas, the dimension of signals is increased by the number of DFT-spread signals as well as the number of users and thus the complexity of the signal detection is further increased.